Pretty sure I won't have time to invest in doing that in the near future, but checking dates on major things listed in the 1.73.0 release notes, and the dates of the merge, it looks like possibly it was either  of these:
```
Update to LLVM 17 (merged 43 days ago)
stabilize abi_thiscall (merged 44 days ago)
```
https://releases.rs/docs/1.73.0/